Every day, a young girl wearing a mask stands by the beach and sings a nostalgic melody. After experiencing two sudden heart-wrenching partings when she was only a child, Nino Arisugawa has been singing her songs to the ocean, bound by a promise made with her two childhood friends—her first love, Momo Sakaki, and a boy who composed music, Kanade "Yuzu" Yuzuriha. Having never met each other, the boys both individually promised that if Nino was ever separated from them, her voice would be the beacon to reunite them once again.